The present invention relates to a hollow body made of an oriented thermoplastic material, such as a bottle or similar article, which exhibits improved mechanical properties and in particular improved impact strength.
It is known that the mechanical properties of hollow bodies made of a thermoplastic can be improved substantially by producing the bodies under such conditions that the molecules of the thermoplastic constituting their walls are oriented, preferably along two orthogonal directions, i.e. are bidirectionally oriented.
In general, in order to achieve this molecular orientation, a preform made from a thermoplastic is first produced, the dimensions of which perform are markedly less than those of the desired oriented hollow body, and subsequently, after having conditioned this preform at a temperature which favors orientation of the thermoplastic by stretching, the desired hollow body is blown, causing longitudinal stretching and radial stretching of the preform.
The oriented hollow bodies thus obtained exhibit a mechanical strength which is markedly improved, to the point that the bodies can generally be used for packaging carbonated drinks under pressure. For this application, it is desirable to impart a suitably chosen shape to the base of the hollow bodies so that their deformation under the effect of the internal pressure in the region of their base does not detract from their stability when standing in an upright, or vertical, position.
Furthermore, it is known that oriented hollow bodies made from a thermoplastic frequently possess an impact strength, in the region of their base, which leaves something to be desired.
Consequently, it has not proven easy to arrive at a shape for the base of hollow bodies made of an oriented thermoplastic which results in both good resistance to deformation of the base and satisfactory impact strength.